Patent GB-638423-A (GB)
Title: Improvements relating to radio receivers capable of selectively receiving angle modulated or amplitude modulated carrier waves and to detector circuits for use therein
IPC: H03D7/10
Filed: 1947-05-19
Abstract: 638,423. Frequency modulation. RADIO CORPORATION OF AMERICA. May 19, 1947, No. 13410. Convention date, May 17, 1946. [Class 40 (v)] A radio receiver, using a frequency - modulation detector of the kind described in Specification 606,007, is switched over. to receive F.M. on one frequency range and A.M. on another, and one or both of the rectifiers used in the F.M. detector is or are also used as the A.M. detector. As shown in Fig. 1, the frequencychanging and I.F, stages 3, 6, 7 are connected alternatively by the switches 4, 5 to a source 1 of short-wave F.M. signals or to a source 2 of A.M. signals on longer waves. The I.F. output taken across a coil 8 is at a much higher frequency for F.M. waves than for A.M. waves, and is applied through condensers 20, 26 to a circuit 11 tuned to the F.M. intermediate frequency. A tap on the circuit 11 is connected to the centre-point of the secondary circuit 12, the ends of which are connected to oppositelypoled rectifiers 13, 14. The discriminator thus formed operates in the manner disclosed in Specification 606,007 and does not require amplitude limitation of the F.M. wave. The detected F.M. signal appears across resistances 28<1>, 30 and is taken off. through condenser 29 connected to their junction. A condenser 34 connected through switch 31<1> across resistor 30 affords de-emphasis of the higher notes. Another switch 24<1> connects resistor 27 and large condenser 24 between rectifier 13 and earth, as in the prior Specification. When used for receiving the longer A.M. waves, the reactances of circuits 15, 17 are negligible, and tuning of the I.F. output is effected by cored coil 8 in parallel with the series condensers 20, 26. Rectifier 13 is inoperative, and the resistors 28<1>, 30 form a shunt load-impedance to the rectifier 14. The de-emphasis condenser 34 is cut out by throwing the switch 31<1> to the dotted A.M. position, and A.V.C. voltage is then taken across resistance 30 through the smoother 41, 43. When receiving F.M., A.V.C. voltage is taken across resistor 27 through switch 24<1> and a tapping on the smoother resistor 41. In a modified circuit, Fig. 3, the anode circuit of the I.F. amplifier includes two resonant circuits 50, 53 tuned respectively to a higher frequency for F.M. reception, and a lower frequency for A.M. reception. Circuit 50 is coupled either magnetically or by a condenser 20 to a similarlytuned circuit 11 which is connected to the midpoint of a third circuit 12 so as to operate with the rectifiers 13, 14 as a discriminator-detector for F.M. waves, audio output being taken from the point 25<1> between the condensers 25, 26. The resonant circuit 53 for the longer A.M. waves is coupled to a similarly-tuned circuit 62 which supplies wave energy through the negligible impedance of circuits 11, 12 to the oppositely-poled rectifiers 13, 14. Switch S 1 connects the load resistances 65, 66 to the point 25<1>, from which the audio signal for A.M. waves may be taken. When receiving A.M. waves, the resistor 27 and condenser 24 are disconnected by switch 71, and a resistor 73 is connected in the A.V.C. lead 40 by operating a switch 72. In a further modification, the F.M. discriminator is of the balanced off-tune type (Fig. 4, not shown). Specification 489,094, [Group XL], and U.S.A. Specification 2,057,640 are referred to.